Combined Sewer Overflows: Detroit’s Water Pollution Problem

The city of Detroit released 3 billion gallons of untreated wastewater into the Detroit River in 2020.

Published on:

September 15, 2022

The entrance to the GWK Retention Treatment Basin. Madison Heights, MI. A red brick building with a green metal roof with green trees in the background.

       The city of Detroit released 3 billion gallons of untreated wastewater into the Detroit River in 2020 1. While releasing sewage, industrial runoff, and ground contaminants into the Great Lakes ecosystem is an ecological disaster; it is a last-ditch effort at preventing the destruction of Detroit’s water treatment system. This is due to a massive design flaw in the over 100-year-old sewers made by combining sewage treatment lines and stormwater runoff into a single pipe. Now known as a combined sewer system (CSS), its implementation has had long-lasting impacts on the city of Detroit, and the Great Lakes region.

With their introduction in the late 19th century, combined sewer systems were a game changer in urban infrastructure. The invention of an underground piping system that transported away household sewage, industrial waste, and excess stormwater on a city-wide scale was unprecedented 2. Before the invention of the CSS, unregulated waste disposal in cities led to large cesspool ditches made of human, animal, and industrial waste. Their overflow when it rained compelled cities to deal with their mess due to the stench. Luckily, Detroit, along with many other rust belt cities in the Great Lakes region, were starting to boom just after this urban planning revelation. While Detroit’s first sewer system was introduced in 1836, its expansion follows the population explosion with the most aggressive sewer construction happening in the early 1900s 3. With such rapid expansion, it was more practical cost-wise to implement one pipe that could hold sewage and runoff together. It is also important to consider that Detroit did not construct its first sewage treatment plant until 1940. Preventing contaminants from reaching the river was not a consideration, as it was the end outcome of the sewer system anyways. The construction of Detroit’s CSS kept the city streets ‘clean’ for the 1900s, but the oversight of a single pipe system is a costly mistake that still haunts Detroit and its taxpayers to this day. 

Today, the Detroit Water and Sewage Department (DWSD) maintains 2,700 miles of transmission and distribution mainlines, along with 3,000 miles of sewer collection piping 4. Any sewage, industrial waste, or rainwater runoff will find its way through this network of pipes to one of Detroit’s five water treatment facilities. There, it is sanitized and filtered before being released into either the Detroit or Rouge River. On most days of the year, the sewer system works as planned – all waste is treated by the facilities and no pollutants reach the river. On days of heavy rainfall, however, the story is much different. While in most cases the sewer pipes can handle the influx of rain, the water treatment plants cannot. Detroit’s largest treatment facility can handle 400 million gallons of waste per day 5. This is more than enough to treat the city's sewage needs, but dauntingly small compared to the amount of water brought down by rainfall in the Great Lakes ecosystem. According to the Great Lakes Water Authority (GLWA), one rainfall event on August 2, 2020, brought down 500 million gallons of rainwater and inundated the sewer system in only two hours 1. Well above their treatment capacity, the DWSD and GLWA had no choice but to release the excess water untreated into the Detroit River in what is known as a combined sewer overflow (CSO).  

Diagram of a combined sewer overflow. Credit: Wayworks Designs.

CSO events are devastating to the health of local ecosystems and communities. As stated in the Environmental Protection Agency’s CSO Policy doctrine, “CSOs often contain high levels of suspended solids, pathogenic microorganisms, toxic pollutants, organic compounds, oil,  grease, and other pollutants.” Oftentimes the pollution is so bad it is possible to visually differentiate between the runoff and the river. These pollutants don’t just dilute away, they persist and feed into toxic algae blooms, “pose risks to human health, threaten aquatic life, and impair the use of the Nation’s waterways 6.” 

A CSO plume where the Rouge and Detroit Rivers meet. Credit: Robert Burns, Detroit Riverkeeper.

I got the chance to speak with a resident from River Rouge who routinely fishes at the riverside Belanger Park and has witnessed these events firsthand. While he wishes to remain anonymous, he spoke about how he spends a lot of time fishing now that he is retired. He doesn’t do it for sport either, he said that he is fortunate enough to get a lot of his meals from the river. While CSO events affect all the wildlife that he catches and eats, he said that he has learned to steer clear of fishing directly after heavy rainfall. “You don’t want to catch anything then,” commenting on the smell of the fish and the river. Unfortunately for him and the rest of the fishermen on the Detroit River, the impacts of climate change most likely mean more fishless days to come. 

Fishermen at Belanger Park along the Detroit River. Original Photograph.

In the 2020 calendar year, there were 452 CSO events reported by the GLWA. Factoring in climate change, the outlook on their frequency does not look bright. From 1981 to 2010, Southeast Michigan saw an average increase of 11% in total precipitation, and the heaviest 1% of rainfall in 1958 now occurs 37% more often 7. Michigan is also experiencing an increase in multi-day heavy rainfall events, which increases the amount of pollution dumped as the capacity is often overloaded on day 1. Future models on temperature do not look promising either, with hotter temperatures meaning more of Michigan's precipitation coming from rain rather than snow 7. A wetter spring, summer, and fall mean increased pressure on the DWSD system. With these factors expected to worsen in the coming decades, Detroit's battle with CSO events will only become more difficult to combat. 

The city of Detroit is taking action to fight these CSOs, but with 20 years of investment, this wicked problem proves costly and difficult to solve. In the case of some neighborhoods with new construction, it is possible to install a separate sanitary sewer system. On the scale of the entire city, however, it would be practically and economically impossible to gut and replace everything. So, the city and its engineers have turned to water retention treatment basins (RTB) to help screen and increase the capacity of the water system when needed. Facilities like the George. W Kuhn RTB, the largest wastewater screening center in North America can screen and partially disinfect over 5 billion gallons of water per day. Serving 14 communities and a drainage area of 24,500 acres 8, the facility greatly reduces the number of contaminants reaching the river while also increasing the capacity of the entire system during downpour events. Additionally, 14 other RTBs are located in metro Detroit, which were able to treat 92% of the 32.5 billion gallons that the GLWA experienced during its 452 CSO events in 2020. While impressive, these RTBs are costly to build, upgrade, and maintain. To date, over 1.4 billion dollars have been dedicated to the stormwater, asset management, and wastewater (SAW) program, and it is estimated that another 200 million dollars will be needed to increase the stormwater capacity to be able to handle that last 8% percent of volume during high-intensity precipitation events 1. This estimate, however, is based on our current precipitation trends and CSO volume data. With an unpredictable future of precipitation looming over the city due to climate change, Detroit could find itself constantly needing hundred-million-dollar upgrades to stay on top of increasing precipitation volume. 

Entrance to the GWK Retention Treatment Basin. Madison Heights, MI. Photo credit: 

      Taking action into their own hands, many Detroit residents are working to beautify the community and reduce the root cause of CSO events through local green infrastructure. 

       I was able to speak with Fai Foen, Director of Green Infrastructure at the non-profit The Greening of Detroit. They have planted over 135,000 trees over the last 30 years and are also participants in the Land+Water WORKS program, a coalition of 10 independent non-profits working together to provide Detroit residents with the resources needed to become better stewards of our environment. “Vegetation and land use is a big factor in CSO events,” Fai said. Impervious surfaces such as concrete and asphalt accelerate the flow rate of drainage into the sewer system. Increasing tree cover, local vegetation, and creating rain gardens all help disperse and hold precipitation while bettering the community at the same time. While greening Detroit is a long-term effort, Fai spoke on a few things that everyone can do at home to help minimize the impact of CSOs. Redirecting your downspouts onto your lawn instead of a hard surface is an easy change you can make that reduces the flow rate and allows some of it to become groundwater. Refraining from doing laundry, showering, or washing dishes during heavy rainfall events also minimizes your household's wastewater output during high volume times. While they may be small changes, if practiced by a majority they could make a big difference in reducing the volume of water the already overloaded system needs to handle. 

Example of a Michigan Rain Garden. Credit: People for Palmer Park Detroit.

       For residents of Detroit, you can also reduce your water bill and earn tax credits by engaging in green infrastructure practices. From the city of Detroit, “Properties can qualify to have charges reduced up to 80 percent through implementation and proper operation of a stormwater management practice depending on the available storage and site conditions.” This can be done by installing a rain garden, reducing impervious acreage on your lot, or installing rain barrels at the end of your drainage system. Not only can you help reduce the impact on CSOs in the local ecosystem, but you can save money in the process. 

       The city of Detroit has come a long way in reducing the volume of overflow experienced during CSOs. For reference, Detroit has reduced its number of untreated discharge events by over 80% since 1988, and with proper funding, plans to eliminate all of them by 2025. While this may be a short-term eradication due to climate change, the city and its residents are working to act responsibly for the betterment of the Great Lakes ecosystem. Anyone living in a drainage zone that is serviced by a combined sewer system can take action, no matter how small, to help combat CSOs. We can all make an impact, whether it’s through volunteering, planting trees, redirecting our house drainage, or being cognizant of high-volume sewer times when doing household chores. Spread the word, take action, and help make an impact in your local environment. 


1. Michigan Department of Environment, Great Lakes, and Energy. Rep. Combined Sewer Overflow (CSO), Sanitary Sewer Overflow (SSO), and Retention Treatment Basin (RTB) Discharge 2020 Annual Report (January 1, 2020 - December 31, 2020). Vol. (January 1, 2020 - December 31, 2020), n.d. 

2. Tibbetts J. Combined sewer systems: down, dirty, and out of date. Environ Health Perspect. 2005 Jul;113(7):A464-7. doi: 10.1289/ehp.113-a464. PMID: 16002363; PMCID: PMC1257666.

3. “DWSD - Wastewater Infrastructure System.” HUBBELL, ROTH & CLARK, INC., March 1, 2021. 

4. “About DWSD.” City of Detroit. 

5. “Facilities - Great Lakes Water Authority.” GLWA, March 12, 2020.'s%20fourth%20water%20treatment%20plant,240%20million%20gallons%20per%20day. 

6. Environmental Protection Agency. Rep. Combined Sewer Overflow (CSO) Control Policy; Notice . Federal Register / Vol. 59, No. 75 / Tuesday, April 19, 1994 / Notices, n.d. 

7. Great Lakes Integrated Sciences and Assessments (GLISA) Center, 2012: Historical Climatology: Michigan Climatic Division 10: Southeast Lower Michigan. Great Lakes Climatic Divisions. D. Brown, J. Andresen, and A. Pollyea Editors. 

8. “George W. Kuhn Drain Detroit, Michigan, USA.” 

9. “Drainage Program Guide - Detroit.” Accessed September 5, 2022. 

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